Methionine adenosyltransferase activity is repressed in rapidly growing tissues such as are found in young animals or tumors. This work will utilize cell culture systems to investigate the hypothesis that this enzyme regulates the balance between proliferation and differentiation by controlling the availability of methionine for protein synthesis. Methionine adenosyltransferase levels and intracellular rates of S-adenosylmethionine synthesis will be measured in mouse 3T3 cells before and after transformation by polyoma virus. Dimethylsulfoxide (DMSO) induces normal morphology and contact inhibition in polyma virus-transformed 3T3 cells and erythroid differentiation, including hemoglobin synthesis, in Friend erythroleukemic mouse cells. I have recently found that DMSO stimulates methionine adenosyltransferase. In order to determine if such stimulation occurs as a prerequisite to DMSO induction, methionine adenosyltransferase levels and intracellular rates of S-adenosylmethionine synthesis will be measured in these cells as function of the degree of response induced by DMSO. The level of charging of methionine tRNA will be measured during DMSO induction as a further test of the above hypothesis. Cycloleucine, which prevents DMSO activation of methionine adenosyltransferase, will be tested in cell cultures to see if it will block DMSO induction of erythroid cell differentiation. Rat liver methionine adenosyltransferase will be isolated and characterized so that the regulatory properties of this enzyme may be more fully understood.